As the radar antenna turns, it emits extremely short bursts of radio waves, called pulses. Each pulse lasts about 0.00000157 seconds (1.57x10 -6 ), with a 0.00099843-second (998.43x10 -6 ) "listening period" in between. The transmitted radio waves move through the atmosphere at about the speed of light.

By recording the direction in which the antenna was pointed, the direction of the target is known as well. Generally, the better the target is at reflecting radio waves (i.e., more raindrops, larger hailstones, etc.), the stronger the reflected radio waves, or echo, will be.

This information is observed within the approximately 0.001-second listening period with the process repeated up to 1,300 times per second. By keeping track of the time it takes the radio waves to leave the antenna, hit the target, and return to the antenna, the radar can calculate the distance to the target.

The WSR-88D's pulses have an average transmitted power of about 450,000 watts. By comparison, a typical home microwave oven will generate about 1000 watts of energy. However, because of the very short period the radar is actually transmitting, when the time of all pulses each hour are totaled (the time the radar is actually transmitting), the radar is "on" for a little over 7 seconds each hour. The remaining 59 minutes and 53 seconds are spent listening for any returned signals.

Courtesy of NWS SRH Jetstream

The Doppler Advantage

By their design, Doppler radar systems can provide information regarding the movement of targets as well their position. When the WSR-88D transmits a pulse of radio waves, the system keeps track of the phase (shape, position, and form) of the transmitted radio waves.

By measuring the shift in phase between a transmitted pulse and a received echo, the target's radial velocity (the movement of the target directly toward or away from the radar) can be calculated. A positive phase shift implies motion toward the radar and a negative shift suggests motion away from the radar.

The larger the phase shift, the greater the target's radial velocity. The phase shift effect is similar to the "Doppler shift" observed with sound waves. An object emitting sound waves will transmit those waves in a higher frequency when it is approaching your location (inbound velocity = positive shift) as the sound waves are compressed. As the object moves away from a location, the sound waves will be stretched and have a lower frequency (outbound velocity = negative shift). You have probably heard this effect when an emergency vehicle drove past you with its siren blaring. As the vehicle passed your location, the pitch of the siren lowered.

While often depicted as a cone with distinct edges, the radar beam is better visualized much like that of ordinary household flashlights. In a darkened room take a flashlight and, while standing 10 feet away or more, shine it on a wall. You will notice the bright area around the center of the beam but will also notice you can see the brightness fade farther away from the beam's center point. You will also notice the width of the beam spreads or decreases as you move toward or away from the wall.

The beam of energy transmitted from the doppler radar is no different. A conical shaped beam is formed as the energy moves away from the radar. And it is near the center line of the beam where most of the energy is located with the energy decreasing away from the centerline.

Volume Coverage Patterns

The WSR-88D employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, or elevation slices, as it rotates. These elevation slices comprise a volume coverage pattern or VCP. Once the radar sweeps through all elevation slices a volume scan is complete. In precipitation mode, the WSR-88D completes a volume scan every 4-6 minutes depending upon which VCP is in effect, providing an updated 3-dimensional look at the atmosphere around the radar site.

The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making multiple 360° scans of the atmosphere, sampling a set of increasing elevation angles.

There are two main operating states of the WSR-88D; Clear Air Mode and Precipitation Mode. Within these two operating states there are several VCPs the NWS forecasters can utilize to help analyze the atmosphere around the radar. These different VCPs have varying numbers of elevation tilts and rotation speeds of the radar itself. Each VCP therefore can provide a different perspective of the atmosphere.

Common among all VCPs (except for VCP 12) is the tilt elevation of the lowest five elevation angles. The scanning begins with 0.5° elevation meaning the centerline the radar beam antenna is angled 0.5° above the ground. Since the the beam itself is 1° wide, it returns information about what it "sees" between 0° and 1° above the horizon. As it completes that elevation scan the radar is tilted another degree with the center line of the beam now at 1.5° and the process of observing the atmosphere begins again then continues through the 2.4°, 3.4° and 4.3° elevation angles.